ia64: convert unwcheck.py to python3

Since my system use python3 as default, arch/ia64/scripts/unwcheck.py no
longer run.

This patch convert it to the python3 syntax.
I have ran it with python2/python3 while printing values of
start/end/rlen_sum which could be impacted by this change and I see no difference.

Fixes: 94a47083

 ("scripts: change scripts to use system python instead of env")
Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>